Hear this sound one time, know what it means, and it can send a wave of fear through you: "Whumph!"

It sounds like somebody just dropped a sack of potatoes from 50 feet high into 3 feet of powder snow.

On instinct, you will turn to see where it came from. There are no indicators: A moonscape of fresh snow extends as far as you can see up the steep mountain slopes. If you're not scared by now, then you don't realize the risk: The daunting power of nature may be about to unleash its ultimate winter force, an avalanche.

The "whumph" noise is a warning sound that an avalanche may be imminent.

It occurs when a deep layer of light, fresh powder piles high atop a dense layer of frozen ice beneath it. The whumph noise is the sound of that powder compressing, shifting or sliding a bit downhill. That's how avalanches get started.

Over the coming holidays and into January, anybody who ventures into the snow country away from resorts should use extreme caution for avalanche danger. This includes those who cross-country ski, snowshoe, telemark free-lance style or ride a snowmobile, especially into mountain bowls that feed up into steep slopes.

Last week, there was a shift in force, as we call it.

Just a week ago, the snow line was as high as 9,000 feet, with heavy rain on top of snow in the high country. The moisture soaked in like a giant sponge. When temperatures plummeted, it then froze into a giant ice block.

The snow level then dropped as low as 3,500 to 4,000 feet in the Sierra foothills with 15 inches (and more in some spots) of powder high in the scoop-like cirques. That light powder piled high on the ice block.

If you were up there, you might have heard a whumph or have seen an avalanche, like the one in Lee Vining Canyon east of Yosemite along Highway 120.

On steep slopes, the light powder can slide down across that ice block, gather in strength and eventually unleash a wall of hurtling snow that plunders everything in its path.

In current conditions, Class 2 slab avalanches, which are large enough to bury a person, are possible in the upper reaches of steep-sloped bowls in wilderness.

Class 3s are big enough to break trees. If it keeps snowing - and storms are forecast to pulse on and off into the New Year - there is a chance for that in some spots, especially canyons and steep, bowl-like areas that receive little sunlight.

Typical danger spots include the Old Ski Bowl high on Mount Shasta, the north-facing slope of Unicorn Peak in Yosemite, the canyon below Taboose Pass in the Eastern Sierra, and many other similar spots. Many are great destinations to trek to the ridge and then sail down on a snowboard or skis, do-it-yourself style.

Two feet of powder sitting on top of a sloped ice block can be plenty to create avalanche danger.

If a slope is facing the wind, huge drifts, or what are called wind slabs, can form. This is common at some ski parks, where ski patrol members will inspect the mountain before dawn and throw small charges into the slabs to release the pressure. If you can't recognize a wind slab when you see one, you should not try to freelance in areas outside of ski parks.

The best advice is to wait until the snowpack stabilizes.

There's an easy way to determine that: Dig a hole in the snow and see how much powder is piled atop the frozen layer, which is often marked by pine needles and twigs. Powder at least 2 feet deep on an ice block can sound a warning.

Wilderness rangers use a more sophisticated method, but anybody can do this:

On a slope where there is fresh powder, isolate a square of snow from 1 to 6 feet across by digging around it into the ice slab so that a square of snow sits as a separate block. Then step on it. If the snow block slides across the ice, avalanche probability is high.

With a failed test, a 25-degree slope in avalanche-prone areas can be enough for danger; 30- and 40-degree slopes, popular with maverick boarders, can be deadly.

And if you go anyway, hoping for the best, and hear the "whumph" noise, it may be too late.

Storm fit for an ark

Marin: On the north flank of Mount Tamalpais, it rained 11 inches in a week, and seven lakes in the watershed are now at 93 percent of capacity.

Foothills: Sterling City on the Feather River had 21.24 inches of rain in six days and Lake Oroville came up 33 feet in the same span, 41 feet overall, and is now 65 percent full (105 percent of normal).

Farther north: It rained 18 inches in six days at Sims along Interstate 5. The Upper Sacramento and McCloud rivers both rose from about 300 cubic feet per second to as high as 30,000 cfs, and the surface of giant Shasta Lake has risen 28 feet since Thanksgiving. It's now 67 percent full (108 percent of normal, despite a drought last year).

Best for outdoors

Remember the survey I did among California outdoors writers who picked the Eastern Sierra as the best place to live if you love the outdoors? In a survey of Lonely Planet travel writers, it came in No. 1 in California and No. 6 in the nation for top travel destination: 1. Louisville, Ky., 2. Fairbanks, Alaska, 3. San Juan Islands, Washington, 4. Philadelphia, 5. American Samoa, 6. Eastern Sierra, California, 7. Northern Maine, 8. Twin Cities, Minnesota, 9. Verde Valley, Arizona, 10. Glacier National Park, Montana.

Outdoor notes

Hawk watch: Raptor migration in the vicinity of Hawk Hill at the Marin Headlands has taken wing, with 8,500 red-tailed hawks and 25,000 raptors verified. Updates: ggro.org

On the inside: In Yosemite, rangers are getting ready to crack down on park employees residing in Yosemite Valley who let their cats outside off leash, potentially leaving them free to kill songbirds. That's right: In a park, you need to keep your cat on a leash or get a ticket.

Plates for powder: Anybody who purchases a new Lake Tahoe license plate gets two free lift tickets to one of 11 Tahoe resorts.

No more slot limit: The Fish and Game Commission voted to end the slot limits for bass at lakes Oroville, McClure, Millerton, Orr and Siskiyou effective March 1. Why? Because bass anglers already release 97 percent of their catch, according to a study, rendering the law irrelevant.

Duck flood: In the counts released this past week, 958,766 ducks and 351,052 geese have taken up residence at the six wildlife refuges that make up the Sacramento National Wildlife Refuge Complex.

Big Fish Club: 410-pound yellowfin tuna for John Peturesco on the boat Excel out of San Diego. ... If you follow my new daily tweet, you know about the 42-inch, 28-pound, 9-ounce mackinaw trout at Lake Tahoe for Eric St. Denis and the 36-inch, 24-pound cutthroat trout at Pyramid Lake for Ed Smith of Reno.